This application claims priority to Japanese Patent Application No. JP 2001-245134, filed on Aug. 13, 2001, the disclosure of such application being herein incorporated by reference to the extent permitted by law.
1. Field of the Invention
The present invention relates to a process of growing a nitride semiconductor and particularly to a process of growing a nitride semiconductor on a nitride semiconductor substrate.
2. Description of Related Art
Nitride semiconductors used in light-emitting devices such as light-emitting diodes and laser diodes, or light-receiving devices such as optical sensors are fabricated by growth on substrates of different materials other than nitride, such as sapphire or silicon carbide.
In some conventional fabrication processes of this kind, a low-temperature amorphous gallium nitride buffer layer is formed on a sapphire or silicon carbide substrate, and then a crystalline nitride semiconductor is grown on the buffer layer. Also, to prevent dislocation defects, the nitride semiconductor on the low-temperature buffer layer is etched in a stripe form for re-growth into another nitride semiconductor. These processes take advantage of the fact that dislocations do not significantly propagate laterally from the underlying nitride semiconductor, i.e., that the re-grown lateral nitride semiconductor is relatively dislocation-free.
Even under the above fabrication process, however, the dislocation problem associated with the growth of nitride semiconductors has not been satisfactorily solved.
Under such situation, thanks to progress in growth processes such as hydride vapor phase epitaxy and metal organic chemical vapor deposition, relatively thick nitride semiconductor crystalline substrates can be fabricated. If nitride semiconductors are grown on these crystalline substrates, the dislocations and other defects will be reduced and cleavage and thermal conductivity may be improved, thus making it expectable to fabricate nitride semiconductors with better crystallinity.
However, conditions for growing nitride semiconductors of optimum characteristics on crystalline nitride semiconductor substrates have not been established yet.
In order to satisfy the above and other needs, the present invention provides a nitride semiconductor growth process which is capable of growing on a crystalline nitride semiconductor substrate a nitride semiconductor having satisfactory crystalline conditions suitable for electronic applications including but not limited to light-emitting and light-receiving devices.
According to a first preferred embodiment of the present invention, a process of growing a nitride semiconductor on a crystalline nitride semiconductor substrate is carried out by heating the substrate and initiating a supply of source gases onto a surface of the substrate before the temperature of the substrate exceeds 1200xc2x0 C. to initiate growth of the nitride semiconductor on the substrate.
According to this preferred embodiment of the present invention, even when the growth temperature for the nitride semiconductor is set at 1200xc2x0 C. or higher, the growth of the nitride semiconductor on the substrate can be initiated when the substrate temperature has reached at least 1200xc2x0 C. This permits the nitride semiconductor growth to start on the substrate surface before development of roughness on that surface due to desorption of nitride semiconductor material from that surface, thereby implementing the nitride semiconductor growth on the substrate having satisfactory surface conditions.
In another preferred embodiment of the present invention, a process of growing a nitride semiconductor on a crystalline nitride semiconductor substrate is carried out by heating the substrate to a temperature not exceeding 1200xc2x0 C. to subject a surface of the substrate to thermal cleaning, and thereafter initiating growth of the nitride semiconductor on the substrate.
According to this preferred embodiment of the present invention, the substrate is thermally cleaned at a temperature not higher than 1200xc2x0 C., while preventing desorption of nitride semiconductor material from its surface. Thus, the thermal cleaning removes oxides from the substrate surface, while preventing desorption of nitride semiconductor material therefrom to keep the substrate in satisfactory surface conditions, thereby allowing the nitride semiconductor growth to start on the optimized substrate surface.